It is required from the contemporary DC to DC switch mode power converters to have high power density, high efficiency and low cost. Certain series resonant type DC to DC power converters, such as the conventional series resonant converter, the LLC converter or the resonance tapped transformer converter described in U.S. Pat. No. 5,907,236 attempt to meet these requirements by providing zero voltage switching operation of power transistors on the primary side of the converter and zero current switching operation of output rectifiers on the secondary side.
Zero voltage and zero current switching are well established switching techniques for reducing switching losses. This allows higher switching frequencies, reduced size of magnetic components, increased power density and reduced cost. Another means of reducing the size of magnetic components is to integrate into the transformer the inductors needed for the normal operation of the converter, such as the resonant, magnetizing and the output filter inductors. The transformer is usually the bulkiest and most expensive component of the circuit.
It is known to employ the leakage inductance between the primary and secondary winding of a transformer as a resonant tank inductance, or in other words to integrate the resonant inductor into transformer's structure. The value of the leakage inductance can be controlled by spacing apart the primary and secondary windings in radial or axial directions as well as by using so called “magnetic shunts”.
It is also known to employ the magnetizing inductance of a transformer for storing energy and extending the load current range featuring zero voltage switching conditions. The value of the magnetizing inductance can be controlled by gapping the transformer and changing the gap dimensions. Such design approach also results in integration of the required magnetizing inductance into transformer's magnetic structure.
A discussion of such “integrated magnetics” design techniques can be found in a text by R. Severns and G. Bloom entitled “Modem DC/DC Switchmode Power Converter Circuits”; (Van Nostrand Reinhold Company, 1985).
Integrated magnetic structures are also described in U.S. Pat. No. 4,262,328 to Bloom, U.S. Pat. No. 5,619,400 to Bowman and U.S. Pat. No. 5,555,494 to Morris.
FIG. 2 illustrates a prior art integrated magnetics converter disclosed in U.S. Pat. No. 5,555,494. The primary winding of the transformer located on the middle, ungapped leg of an E-shape transformer core is powered by a full wave, pulse width modulation (PWM) controlled converter. The primary winding induces flux in the transformer core, so that two secondary windings provide current to the load. Each of the secondary windings is located on a gapped side leg of the E-shape magnetic structure and performs smoothing (filter) inductor function in addition to its conventional, secondary voltage source function. Such integration of the filter inductor in the secondary winding provides inductively filtered output and is therefore not suitable for series resonant type DC to DC power converters that require capacitively filtered output. The presence of a filter inductor in the output rectifier path seriously disturbs the operation of series resonant type converters and eliminates some of their advantageous characteristics, such as reduced voltage stress and reduced switching losses in output rectifiers. An output filter inductor in series resonant type converters can only be employed if the converter output has already been capacitively filtered, i.e. such inductor can only be connected between an output filter capacitor and the load impedance.
A further disadvantage of Morris's converter is the need to double the number of turns of the secondary winding when moving it from the center leg of the transformer to the side legs. This leads to increased copper losses in the secondary winding not only because of the increased wire length but because of increased eddy currents losses as well, especially if the number of turns needed cannot be wound in a single layer. Another disadvantage of Morris's converter is the significantly reduced magnetic coupling and increased leakage inductance between the spaced apart halves of the center tap secondary which results in voltage spikes (due to magnetic field energy stored in this leakage inductance) when output rectifiers commutate the load current. Heavy snubbing is usually needed to eliminate these spikes resulting in increased power dissipation and reduced power conversion efficiency.